One type of implantable device is a synthetic vascular graft such as is commonly used to replace damaged or dysfunctional arterial or venous pathways, for example at the site of an aneurysm or occlusion. Bypass grafts are often used to divert blood flow around damaged regions to restore blood flow. Another use of vascular prostheses is for creating a bypass shunt between an artery and vein, specifically for multiple needle access, such as is required for hemodialysis treatments. Following multiple percutaneous invasions into a vein, the vein may either collapse along the puncture track or become aneurysmal, leaky or fill with clot, causing significant risk of pulmonary embolization. Vascular prostheses have been used for many years as an alternative to patients' own veins for vascular access during hemodialysis.
Materials have been developed which exhibit the characteristics desirable for use in artificial prostheses. These characteristics include chemical inertness and resistance to undesirable physical changes in the medium of use. An example of such material is polytetrafluoroethylene (PTFE), a porous polymeric material which may be stretched to a specified length and thickness. When thus expanded, the material consists of a network of interrelated nodes and fibrils. The diameters of the fibers and the size of the pore vary depending on the stretching conditions. Since the porosity of the tubing can be varied, it is possible to adjust the porosity to minimize the occurrence of thrombosis. It is difficult, however, to elicit natural occlusion of suture holes in vascular prosthesis made of PTFE tubing alone, due to the relative elasticity of the porous PTFE tubing material.
Typically, PTFE vascular grafts cannot safely be used to withdraw blood until they have been in place for a minimum of 14 days after surgery and have become surrounded by fibrotic tissue, because of the bleeding which occurs at the site of a needle puncture in these grafts if fibrotic tissue is absent. Complications commonly encountered with early puncturing of PTFE arteriovenous fistulas include a hematoma surrounding the graft, false aneurysm, and graft occlusion. Other materials which have been used for vascular grafts include autologous saphenous vein, woven or knitted Dacron.RTM. brand polyester, or other synthetic polyester fiber, microporous Teflon.RTM., modified bovine carotid xenograft, and modified human umbilical vein. None of these has overcome the problems with early puncture of the graft following implantation.
Another drawback of the vascular grafts described above is that after implantation they initially acquire a thin inner coating of thrombus, which is replaced to some extent by endothelium growing in from the end of the graft or from capillary tufts that penetrate the interstices of the graft. Thus, the degree of porosity is generally a compromise between the capacity of the particular graft to accommodate ingrowing connective tissue and capillary tufts, and excessive permeability to blood at the time of implantation.
In the course of replacing or by-passing damaged arteries and veins, the need often arises to have prosthetic devices of different diameters. In the physical setting, arteries are generally smaller in diameter than veins, thus requiring arterial and venous grafts to be available in a range of diameters. One way to provide grafts of different diameters to the surgeon at the time of implantation is to provide an array of individual vascular grafts each having a different diameter to the surgeon. These grafts are of a fine material, which can become entangled or torn during handling. In order to make these individual grafts communicate with each other, they must be sutured together.
It is therefore, an object of the invention to provide a vascular graft including at least two lumen-defining structures which can be manually separated from one another.
It is another object of the invention to provide such a graft wherein the lumina defined by the structures are of unequal diameters.